Hydraulic drawing cushions have been used on drawing presses for a long time. During a drawing operation the drawing press clamps a piece of sheet metal between a first tool and a second tool. The two tools are moved relative to each other during a working stroke. In the course of the relative movement, the metal sheet is drawn over a tool mold and pressed between two tool molds of the two tools. In the course of drawing the piece of sheet metal the drawing cushion provides the necessary holding force for the metal sheet.
Such a hydraulic drawing cushion has been known from publication DE 10 2006 058 630 A1, for example. The hydraulic drawing cushion comprises a hydraulic cylinder with a cylinder housing and a piston that can be moved back and forth therein. The piston divides the interior of the cylinder into two work chambers. A first piston area is adjacent the first work chamber and a second piston area is adjacent the second work chamber. A hydraulic medium can be filled into the first or the second work chamber by means of a hydraulic pump in order to move the piston rod and the metal sheet holding ring and to set the metal sheet holding force, respectively.
In conjunction with this, it is a problem that the hydraulic pump and the motor driving the hydraulic pump must provide a large volume flow for a rapid movement of the metal sheet holding ring. The work chambers and the piston areas cannot be arbitrarily configured to be small because the hydraulic pressure could then not generate the sufficient metal sheet holding force. If high accelerations or speeds of the piston or the metal sheet holding ring are required, the motor/pump unit must be dimensioned displaying the appropriate power, thus making the hydraulic drawing cushion considerably more expensive.
In order to avoid this disadvantage, publication DE 10 2010 019 324 A1 suggests that a further, third, work chamber be provided in the cylinder, said work chamber being adjacent to a third piston area. In this embodiment, either the piston rod is moved by the appropriate application of hydraulic pressure to the first or second work chamber, or a holding force is generated via the third work chamber during the drawing operation. This embodiment already has resulted in a few advantages in view of the dimensioning of the motor/pump unit.